Air-core transformer



Dec. 6, 1960- w. w. SALISBURY 6 AIR-CORE TRANSFORMER Filed Feb. 13, 195Bhay d 9 a e a Mm n a a 2 i 1d M m M T Z W H 9 T B m u 3 MM n1. H T. Tl aI w ww. H P

United States Patent AIR-CORE TRANSFORMER Winfield W. Salisbury, PaloAlto, Calif., assignor to Zenith Radio Corporation, a corporation ofDelaware Filed Feb. 13, 1958, Ser. No. 715,077

9 Claims. (Cl. 336-192) This invention relates in general totransformers and is particularly directed to transformers of theair-core type. The expression air-core transformer as used herein and inthe appended claims means a transformer which has a core of non-magneticand non-conductive material whether the core, in fact, be air or aninsulating material employed to strengthen the structure mechanically.

Transformers, of course, have a myriad of applications and thedesideratum in their design and construction has always been increasedeificiency or greater and greater coupling coefficients of the windingswith respect to one another. Two well known expedients have beenresorted to in the past for the purpose of improving the transformeraction: (1) the use of a magnetic core to concentrate the magnetic fluxresulting from current flow in the windings and (2) the use of tunedwindings. Neither is entirely satisfactory. Magnetic cores areobjectionable because they introduce losses, specifically hysteresislosses, and they are subject to saturation which undesirably limits thecurrent capacity of the transformer. Moreover, they impose frequencylimitations especially for high-frequency installations. Tuned windingssimilarly impose severe frequency limitations and may be likened to abrute force method of increasing transformer action. The improvementresults from resonance effects rather than from any increase inefiiciency or co-efiicient of coupling.

Transformers of the air-core type do avoid certain of the objectionablefeatures of those employing magnetic cores but, as previouslyconstructed, they have exhibited poor coupling coefficients and have notrepresented any real solution to the quest for a high-eificiencytransformer. High efficiency, which is another expression for a highdegree of coupling, is obviously desirable for any installation, but isessential in systems where the performance depends upon the attainmentof fast circuitry; for example, in the field of controlled thermonuclearreaction.

Efforts have been made to obtain a thermonuclear reaction by confining aplasma of a suitable reactant within :an exceedingly concentratedmagnetic field fashioned to enclose a reactoin space and by pouringenergy into that space. It is necessary in an apparatus pursuing thisapproach to nuclear reaction to supply enormous quantities of energy inshort pulse intervals which may be in :the order of 50 microseconds orless and that can be accomplished only through fast circuits. Suchcircuits are characterized by the fact that their inductance has beenreduced as nearly as possible to an essential minimum.

In practice, the energy is delivered from condenser banks and condensershave been developed which have .a very low internal inductance. Such acondenser is described and claimed in a copending application, SerialNo. 711,376, filed January 27, 1958, in the name of Winfield W.Salisbury, and assigned to the same assignee :as the present invention.Techniques have also been perfected for minimizing the inductance of theleads or conductors employed in constructing the system and a switch forcontrolling current flow in the system, while of itself contributing aminimum of inductance, is the subject of another copending application,Serial No. 715,000, filed February 13, 1958, in the name of Leigh CurtisFoster and, likewise, assigned to the same assignee as the presentinvention. The remaining component of the system, which can be a majorsource of unwanted inductance, is the transformer desired to be employedfor the purpose of achieving currents of extreme intensity in themagnetic structure. Transformers heretofore employed for this purposehave been of more or less conventional construction, exhibitingunnecessary leakage inductance and impairing optimum performance of thesystem by degrading its rise time.

It is an object of the present invention, therefore, to provide atransformer of the air core type particularly useful in high frequencyapplications.

It is another object of the invention to provide an air core transformerhaving a minimum of leakage inductance and exhibiting an extraordinaryhigh coefficient of coupling and efficiency.

It is a specific object of the invention to provide an air coretransformer having an improved coupling coefiicient achieved primarilythrough geometrical or physical considerations of the structure itselfas distinguished from the aid of magnetic cores or tuned windings.

A high frequency air core transformer, constructed in accordance withthe invention, comprises a first winding including a plurality ofhelically wound turns of a flatribbon conductor having a width very muchgreater than its thickness and having a minimal spacing between turns.There is a secondary winding in concentric relation to the firstwinding, formed of a conductor having a width approximately equal to theaxial length of the first winding, to constitute with the first windingan air core transformer having a winding thickness much less than thecore diameter. Insulating material is interposed between the turns ofthe winding and a pair of terminal strips connect to opposite ends ofthe first winding. These terminal strips are formed of conductors havinga width substantially greater than the conductor width of the firstwinding and extending in closely apposed mutually parallel relation. Asecond pair of terminal'stripsis connected to the opposite ends of thesecond winding, having a width much greater than their thickness andlikewise extending from the second winding in closely apposed mutuallyparallel relation.

The features of the present invention which'are believed to be novel areset forth with particularity in the appended claims. The organizationand manner of operation of the invention, together with further objectsand advantages thereof, may best be understood by reference to thefollowing description taken in connection with the accompanyingdrawings, in the several figures of which like reference numeralsidentify like elements, and in which: a

Figure 1 is a perspetcive view of a transformer embodying the teachingsof the invention; a

Figure 2 is an end view of the structure of Figure l; and

Figure 3 is a schematic representation of an electrical system to whichthe transformer may be applied with a special advantage,

Referring now more particularly to Figures 1 and 2, the structure thererepresented is an air core transformer which is especially suited forhigh frequency application. When employed in an electrical system inwhich the trans former is interposed between a condenser bank and amagnetic structure for the purpose of establishing intense magneticfields in response to the discharge of the condenser bank, the dischargeis of'an' oscillatory type" and the frequency corresponds tothe'frequency of'the Patented Dec. 6, 1960 oscillatory discharge. Inthermonuclear systems this frequency is high, being in the neighborhoodof one megacycle per second.

While the transformer may have a strictly air core, it is convenient toform the windings about a core 16 of nonmagnetic and nonconductivematerial but being mechanically strong in order to impart mechanicalstrength to the transformer structure. Such a core may be formed of acanvas base Bakelite, Lucite or other nonmagnetic and nonconductivematerial. Its axial length is preferably somewhat greater than thedesired length of the transformer windings and, while taking the form ofa hollow cylinder, it has a slot 11 extending completely along itslength for a purpose to be made clear hereinafter. There is a winding 12formed about core to constitute one coil turn which is almost but notquite complete. The winding is made of a conductor having a widthapproximately equal to the desired axial lengths of the transformerwindings. Electrically this winding may conveniently be employed as thesecondary of the transformer and the free ends of the conductor fromwhich it is Wound are folded or bent across the coil turn, extendingthrough slot 11 of core 10 and arranged in closely apposed mutuallyparallel relation. These end portions of the conductor are designated 13and 14 and a layer 15 of insulating material is interposed between them.

A layer 16 of insulating material is wound over secondary winding 12 soas to be interposed between the primary and secondary windings.Preferably, layer 16 has a minimal thickness, that is to say, itsthickness is as small as the electrical ratings and properties of thetransformer permit. A layer thickness corresponding to one skin depth inthe winding conductors at the operating frequency is desirable becauseit permits a condition of image currents in close physical proximity tobe established in the primary and secondary windings.

The primary winding is formed over insulating layer 16 and includes aplurality of helically wound turns 17, 17 of a conductor having a Widthseveral times greater, than its thickness, and a spacing between turnswhich is as small as it can be made compatible with the electricalrequirements of the winding. The primary and secondary windings may beformed of conductors having like thicknesses and their thicknessdimension is selected with a view to obtaining an optimum compromise inrespect of the magnetic properties, the electrical resistance and themechanical strength of the windings.

In general, the thinner the winding conductor, the better is itsmagnetic property but its resistance is smallest for a thicknesscorresponding to one skin depth at the operating frequency. Skin depthis defined as follows:

where A is the skin depth;

K is the constant;

p is the resistivity of the winding conductors;

f is the operating frequency; and

u is the permeability of the winding conductor.

Of course, the mechanical strength of the winding increases With thethickness of the conductor and this is seen to be in conflict with therequirement for best magnetic properties. For the case underconsideration the primary and secondary windings are formed ofconductors having a thickness of the order of one skin depth at theoperating frequency.

Terminal strips connect to opposite ends or to the first and last turnsof primary winding 17 and are themselves constructed to have a minimuminductance. To that end, the strips 18, 19 are formed from conductivestrips having a width substantially greater than the width of theconductor from which the primary is formed and,

additionally, the terminal strips extend in closely apposed mutuallyparallel relation, being insulated one from the other by a suitablelayer 20 of insulation. To have the necessary cancellation of leadinductance, strips 18 and 19 should have a width exceeding half theaxial length of the windings but preferably, and as represented inFigure 1, their width corresponds essentially to that of the lengths ofthe windings. Of course, each terminal strip, while being mechanicallyand electrically connected to an end turn of the primary is otherwiseinsulated from the winding turns. Where the terminal strips are formedfrom the same stock as the primary winding, their thickness will beapproximately one skin depth.

As will be explained presently, the desired condition of low leakageinductance requires that currents flowing in the transformer structure,including its lead-ins, be in close physical proximity to imagecurrents. To achieve that condition, at least so far as primary lead-ins18, 19 are concerned, these lead-ins are to have the same size and shapeand to be positioned for complete overlapping as represented in Figure1.

It is of course essential that terminal strips be provided for makingelectrical connections to the inner or secondary winding of thetransformer and this is accomplished by extending terminal sections 13,14 of the secondary conductor, extending them axially from the windingas shown at 23, 24 in Figure 1. Terminal strips 23, 24 have a width muchgreater than their thickness and, more particularly, have a Widthapproximately equal to the core diameter of the transformer. They arepositioned in closely apposed mutually parallel relation and areinsulated from one another by the insulating layer 15.

In general, the desirable properties of a transformer are that it becapable of transforming alternating current electric power from onevoltage-current ratio to another Without excessive power loss and with aminimum of interference in the phase relations which depend on the powerutilizing load. For the described structure, the primary lead-ins 18, 19represent a high voltage and relatively low current condition which maybe transformed through the transformer action to a relatively lowvoltage but intense current available at the terminal strips 23, 24 ofthe secondary. The optimized value of conductor thickness, namely, athickness in the order of one skin depth at the operating frequency,assures minimum power loss in the transformer itself While otherphysical characteristics of the structure contribute to anextraordinarily high coefiicient of coupling or transformer efiiciency.

The energy stored in common by a given current so as to be available toeither winding of the transformer represents the energy which can betransferred from one to the other in each cycle and, accordingly,represents the open circuit or excitation inductance of the transformerwhile the energy stored in such a way as to be available to only onecoil and not to the other represents leakage inductance. The ratio ofleakage inductance to excitation inductance must be minimized to achievehigh efiiciency and coupling coefiicients approaching unity. Primaryconductor 17 is formed into a coil around a large conducting cylinder 12so that the currents which flow in the cylinder are equal and oppositeto those in the exciting coil and may be thought of as images of theexciting current. Such image currents are always so distributed as toreduce to a minimum possible value magnetic flux below the surface ofthe conducting cylinder. If the exciting conductors are close to thesurface from which they induce images in terms of their width andifaction through considerations of its physical or geometricalconstruction as distinguished from prior art techniques wherein enhancedtransformer action is accomphshed through the use of magnetic coresand/or tuned windings with the limitations incident to such techniquesdiscussed hereinabove.

One embodiment of the transformer structure which has been successfullyemployed had the following specification which, being given solely forthe purpose of illustration, represents no limitation of the invention:

Primary conductor 1'? A copper stri 1% inches wide and .04 inch thick.

Interturn spacing of the primary Insulation 16 Secondary conductor 12 ofan inch.

.010 inch of Mylar."

A copper strip having a length equal to that of the primary and the samethickness as primary conductor 17.

6% inches.

4 kilovolts.

200.000 a-mperes in a pulse interval of 50 microsec- !Coefiicient ofcoupling Turns ratio of primary to seeon ary Axial length of transformerwindings 12 inches.

Coefficients of coupling as high as 96% have been achieved withtransformer constructions of the type described.

An electrical system characterized by fast circuitry and employing atransformer in accordance with the instant invention is representedschematically in Figure 3. It includes a high voltage DC. power supply30 of any conventional construction, coupled through a current limitingresistor 31 to an artificial transmission line constituted ofseries-connected inductors 32, 32 and parallelconnected condensers 33,33. The output terminals of the transmission line are connected througha switch 34, represented as electrodes defining a spark-gap, to atransformer 12, 17 which may be considered to be the structurerepresented in Figures 1 and 2. The secondary winding of the transformerconnects to a load circuit 35. The load circuit may take any of avariety of forms and may, if thought of as a component of athermonuclear reaction apparatus, be a magnetic structure for creatingan intense magnetic field in response to the application of highintensity current pulses.

In operating intervals in which switch 34 is open, supply 30 chargescondensers 33 to a high potential through current limiting resistor 31.When the switch is closed through the expedient of establishing an arebetween its electrodes, the transmission line discharges through thetransformer 12, 17. As a consequence, and in virtue of the currentstep-up properties of the transformer, a pulse of current of very highintensity is applied to the load 35.

A system of this type renders its intended performance only to theextent that it is composed of fast circuits so as to have very sharprise times. This necessary attribute may be realized by constructingcondensers 33 in the manner described in the above-identifiedapplication of W. W. Salisbury and by employing a low inductance switch,of the type described in the aforeidentified Foster application, atcomponent 34. The low inductance condensers, low inductance switch andthe current transformer of high coupling coefiicient in minimizedleakage inductance yield very sharp rise times for the system.

While particular embodiments of the invention have been shown anddescribed, it will be obvious to those skilled in the art that changesand modifications may be made without departing from the invention inits broader aspects, and, therefore, the aim in the appended claims isto cover all such changes and modifications as fall within the truespirit and scope of the invention.

I claim:

1. A high-frequency air-core transformer comprising: a first windingincluding a plurality of helically-Wound turns of a fiat-ribbonconductor having a width much greater than its thickness and having aminimal spacing between turns; a second winding in concentric relationto said first winding, formed of a conductor having a widthapproximately equal to the axial length of said first winding toconstitute with said first winding a transformer having a windingthickness much less than the core diameter; insulating material betweensaid windings; a pair of terminal strips connected to opposite ends ofsaid first winding formed of conductors having a width substantiallygreater than the conductor width of said first winding and extending inclosely-apposed mutually-parallel relation; and a pair of terminalstrips connected to the opposite ends of said second winding, having awidth much greater than the thickness and extending from said secondwinding in closely-apposed mutually-parallel relation.

2. A high-frequency air-core transformer comprising: a first windingincluding a plurality of helically wound turns of a flat-ribbonconductor having a thickness of the order of one skin depth at theoperating frequency, having a width much greater than its thickness andhaving a minimal spacing between turns; a second winding in concentricrelation to said first winding, formed of a conductor having a widthapproximately equal to the axial length of said first winding toconstitute with said first winding a transformer having a windingthickness much less than the core diameter; insulating material betweensaid windings; a pair of terminal strips connected to opposite ends ofsaid first winding formed of conductors having a width substantiallygreater than the conductor width of said first winding and extending inclosely-apposed mutually-parallel relation; and a pair of terminalstrips connected to the opposite ends of said second winding, having awidth much greater than the thickness and extending from said secondwinding in closely-apposed mutually-parallel relation.

3. A high-frequency air-core transformer comprising: a first windingincluding a plurality of helically-wound turns of a flat-ribbonconductor having a thickness of the order of one skin depth at theoperating frequency, having a Width much greater than its thickness andhaving a minimal spacing between turns; a second winding in concentricrelation to said first winding, formed of a conductor having a widthapproximately equal to the axial length of said first winding and athickness of the order of one skin depth at the operating frequency, toconstitute with said first winding a transformer having a windingthickness much less than the core diameter; insulating material betweensaid windings; a pair of terminal strips connected to opposite ends ofsaid first winding formed of conductors having a width substantiallygreater than the conductor width of said first winding and extending inclosely-apposed mutually-parallel relation; and a pair of terminalstrips connected to the opposite ends of said second winding, having awidth much greater than the thickness and extending from said secondwinding in closely-apposed mutually-parallel relation.

4. A high-frequency air-core auto-transformer comprising: a firstwinding including a plurality of helicallywound turns of a flat-ribbonconductor having a thickness of the order of one skin depth at theoperating frequency, having a width much greater than its thickness andhaving a minimal spacing between turns; a single-turn second winding inconcentric relation to said first Winding, formed of a conductor havinga width approximately equal to the axial length of said first windingand a thickness of the order of one skin depth at the operatingfrequency, to constitute with said first winding a transformer having awinding thickness much less than the core diameter; insulating materialbetween said windings; a pair of terminal strips connected to oppositeends of said first winding formed of conductors having a widthsubstantially greater than the conductor width of said first winding andextending in closely-apposed mutually-parallel rela tion; and a pair ofterminal strips connected to the op posite ends of said second winding,having a width much greater than the thickness and extending from saidsecond winding in closely-apposed mutually-parallel relation.

5. A high-frequency air-core transformer comprising: a first windingincluding a plurality of helically-wound turns of a flat-ribbonconductor having a width much greater than its thickness and having aminimal spacing between turns; a second winding in concentric relationto said first winding, formed of a conductor having a widthapproximately equal to the axial length of said first winding toconstitute with said first winding a transformer having a windingthickness much less than the core diameter; insulating material betweensaid windings; a pair of terminal strips connected to opposite ends ofsaid first winding formed of conductors having a width substantiallygreater than one-half the axial length of said windings and extending inclosely-apposed mutually-parallel relation; and a pair of terminalstrips connected to the opposite ends of said second winding, having awidth much greater than the thickness and extending from said secondwinding in closely-apposed mutually-parallel relation.

6. A high-frequency air-core transformer comprising: a first windingincluding a plurality of helically-wound turns of a flat-ribbonconductor having a Width much greater than its thickness and having aminimal spacing between turns; a second winding in concentric relationto said first winding, formed of a conductor having a widthapproximately equal to the axial length of said first winding toconstitute with said first winding a transformer having a windingthickness much less than the core diameter; insulating material betweensaid windings; a pair of terminal strips connected to opposite ends ofsaid first winding formed of conductors having a width approximatelyequal to the axial length of said windings and extending inclosely-apposed mutually-parallel relation; and a pair of terminalstrips connected to the opposite ends of said second winding, having awidth much greater than the thickness and extending from said secondwinding in closely-apposed mutually-parallel relation.

7. A high-frequency air-core transformer comprising: a first windingincluding a plurality of helically-wound turns of a fiat-ribbonconductor having a width much greater than its thickness and having aminimal spacing between turns; a second winding disposed within and inconcentric relation to said first winding, formed of a conductor havinga width approximately equal to the axial length of said first winding toconstitute with said first winding a transformer having a windingthickness much less than the core diameter; insulating material betweensaid windings; a pair of terminal strips connected to opposite ends ofsaid first winding formed of conductors having a width substantiallygreater than the conductor width of said first winding and extending incloselyapposed mutually-parallel relation; and a pair of terminal stripsconnected to the opposite ends of said second winding, having a widthmuch greater than the thickness and extending axially of said secondwinding in closely-apposed mutually-parallel relation.

8. A high-frequency air-core transformer compn'sing: a first windingincluding a plurality of helically-wound turns of a flat-ribbonconductor having a width much greater than its thickness and having aminimal spacing between turns; a second winding disposed within and inconcentric relation to said first winding, formed of a conductor havinga width approximately equal to the axial length of said first winding toconstitute with said first winding a transformer having a windingthickness much less than the core diameter; insulating material betweensaid windings; a pair of terminal strips connected to opposite ends ofsaid first winding formed of conductors having a width substantiallygreater than the conductor width or" said first winding and extending inclosely-apposed mutually-parallel relation; and a pair of terminalstrips connected to the opposite ends of said second winding, having awidth approximately equal to the core diameter of the transformer andextending axially of said second winding in closely-apposedmutually-parallel relation.

9. A high-frequency air-core auto-transformer comprising: a firstwinding including a plurality of helicallywound turns of a fiat-ribbonconductor having a thickness of the order of one skin depth at theoperating frequency, having a width much greater than its thickness andhaving a minimal spacing between turns; a single-turn second windingdisposed within and in concentric relation to said first winding, formedof a conductor having a width approximately equal to the axial length ofsaid first winding and a thickness of the order of one skin depth at theoperating frequency, to oonsttiute with said first winding a transformerhaving a winding thickness much less than the core diameter; insulatingmaterial between said windings; a pair of terminal strips connected toopposite ends of said first winding formed of conductors having a widthapproximately equal to the axial length of said windings and extendingin closely-apposed mutuallyparallel relation; and a pair of terminalstrips connected to the opposite ends of said second winding, having awidth approximately equal to the core diameter of the transformer andextending axially of said second winding in closely-apposedmutually-parallel relation.

References Cited in the file of this patent UNITED STATES PATENTS

